1. Field of the Invention
The present invention relates to a toner for use in an image forming method intended for visualizing an electrophotograph, or an electrostatic charge image.
2. Description of the Related Art
A large number of image forming methods such as electrostatic recording methods, magnetic recording methods, and toner jet methods have been conventionally known. For example, a large number of electrophotographic methods have been known (see, for example, U.S. Pat. No. 2,297,691, JP 42-23910 B, and JP 43-24748 B). Electrophotographic methods generally involve: utilizing a photoconductive substance first to form an electrostatic latent image on a photosensitive member of various means; next, developing the latent image with toner to provide a visible image; transferring the toner onto a transfer material such as paper as required; and then fixing the toner image onto the transfer material by using heat, pressure, or the like to provide a copied article. The toner remaining on the photosensitive member without being transferred is cleaned by various methods, and then the above-mentioned steps are repeated.
Various methods and device have been developed for the step of fixing the toner image onto a sheet such as paper, as the above-mentioned final step. A method commonly used in recent years is a pressure and heat fixing system using a heat roller. The pressure and heat fixing system using a heat roller involves passing the transfer material while bringing a surface of the heat roller having releasability with respect to toner and a surface of a toner image on the transfer material into contact with each other under pressure to fix the toner image. In the mode, the surface of the heat roller and the toner image on the transfer material are brought into contact with each other under pressure, so the mode provides extremely good thermal efficiency upon fusion of the toner image onto the transfer material, and allows fixation to be quickly performed.
By the way, various different toners have been used in copying machines and printers depending on models of the copying machines and the printers. This is mainly due to differences in fixation speed and fixation temperature. In other words, the reason for the foregoing is that fixability or a gloss of a fixed image is largely affected by the fixation speed or temperature because the surface of the heat roller and the toner image in a molten state are brought into contact with each other under pressure. General setting is as follows: the temperature of the surface of the heat roller is set to be low when a fixation speed is low, while the temperature of the surface of the heat roller is set to be high when the fixation speed is high. This is due to a need for keeping a quantity of heat to be applied by the heat roller to the toner at a nearly constant value irrespective of the fixation speed in order that the toner may be fixed onto the transfer material.
Under such circumstances, a user's preference for, in particular, gloss varies from image to image, so it has been desired that the user should be capable of selecting a level of glossiness. A relationship between a loss modulus G″ and a temperature at which the loss modulus is obtained has been described as an approach for controlling the gloss (see, for example, JP 2002-099110 A and JP 2004-151438 A). However, a temperature region in which desired gloss can be obtained in each of the documents cannot be said to be sufficient, and is still susceptible to improvement.
In addition, when the quantity of heat to be applied onto the transfer material varies, the gloss of an image to be obtained also varies. Suppose, for example, the following case: when the transfer material is passed through a fixing unit, the temperature of a heat roller decreases so that the quantity of heat to be applied to a leading end portion of the transfer material at the time of onset of fixation is different from the quantity of heat to be applied to a rear end portion of the transfer material at the time of completion of the fixation. In this case, the end portions of the resultant image are different from each other in gloss. One is apt to have a feeling of discrepancy in a case of a full-color image in particular. Further, when a large number of images are continuously outputted, the temperature of the heat roller decreases, with a result that an image at the time of onset of the image output and an image to be obtained at the time of completion of the image output are different from each other in gloss in some cases.
In addition, a fixing device according to a film heating mode has been recently put into practical use from the viewpoints of quick start property and energy savings (see, for example, JP 04-44075 A and JP 04-204980 A). However, the fixing device is problematic in terms of quantity of heat when the device is used for a full-color image forming device or a high-speed machine, each of which requires a large quantity of heat. In addition, in the fixing device, applied pressure of a fixing nip portion cannot be made very large, so the device is problematic in terms of improvements in gloss and speed.
Further, a toner has been desired which not only allows a toner image to be certainly brought into close contact with paper but also prevents a fixing roller from generating, for example, an offset upon fixation, and which does not cause any image defect due to a stress after the fixation.
Meanwhile, a method of increasing a degree of branching of a polymer of a vinyl resin by using a macromonomer (see, for example, JP 03-87753 A and JP 03-203746 A), and a method involving use of a hydroxy vinyl monomer modified with ε′-caprolactone (see, for example, JP 04-24648 A) have been proposed. However, when a large number of macromonomers are used for increasing the degree of branching in each of those methods, a glass transition temperature of the resin decreases, and storage stability of the toner reduces. Examples of a method of keeping the storage stability of the toner at a good level include: a method involving increasing molecular weight of a main chain of the polymer to increase the glass transition temperature of the main chain of the polymer; and a method involving changing a monomer composition of the main chain of the polymer without changing the molecular weight of the main chain to increase only the glass transition temperature of the main chain. However, any of those methods increase the fixation temperature. In addition, a lowering effect of the temperature at which the toner is fixed due to an increase in the degree of branching is small, so an improvement in gloss is difficult to be obtained.
A method involving the use of: a polyvalent carboxylic acid which is trivalent or more or a polyhydric alcohol which is trivalent or more; or a dicarboxylic acid having a side chain or a diol having a side chain has been proposed as a method of increasing the degree of branching of a polyester resin (see, for example, JP 59-228658 A and JP 62-195678 A). However, the method involving the use of a polyvalent carboxylic acid which is trivalent or more or a polyhydric alcohol which is trihydric or more increases an amount of a gel component (i.e., THF insoluble matter), though the method increases the degree of branching. As a result, hot offset resistance is improved, but a fixation temperature is increased.
A relationship between weight average molecular weight measured by a light scattering method and weight average molecular weight measured by GPC of a component showing a molecular weight of 50,000 or less in a molecular weight distribution by GPC in a THF soluble matter of a toner binder resin has been described (see, for example, JP-09-6050 A). In addition, a relationship between a weight average molecule weight measured by a light scattering method and an inertial radius in a toner binder resin having a molecular weight in a region of 2,000 to 100,000 has been described (see, for example, JP 09-146305 A). Further, a toner containing the low-molecular-weight polymer component whose molecular weight is 50,000 or less and, whose branching index is less than one has been described (see for example, JP 07-230188A). However, the toner binder resin described in each of those documents has a low degree of branching and a limited molecular weight range. Accordingly, the resin cannot sufficiently take balance of low-temperature fixability, offset resistance, and night gloss at a high level.
Further, the respective color toners mounted on a full-color copying machine or printer must be sufficiently mixed in a heat pressure fixing step without an impairment of an improvement in color reproducibility or transparency of an overhead projector (OHP) image.
For example, a toner containing a linear polymer excellent in low-temperature fixability, gloss, and OHP image transparency as the binder resin has been described (see, for example, JP 2002-07255 A). However, the toner is still susceptible to improvement in a recent high-speed developing system.
As described above, in a high-speed developing system, a toner has been desired which provides a high-quality image without causing any image defect due to a stress after fixation, and which provides an image having a constant gloss width in a wide temperature region irrespective of the constitution of a fixing unit.